1. Field of the Invention
The invention disclosed herein relates to subterranean imaging and, in particular, to improvement of electrode responses providing for resistivity imaging within a wellbore.
2. Description of the Related Art
Imaging of formations surrounding boreholes provides valuable information for describing geologic features. Some of the features include structural framework, fracture patterns, sedimentary features, and in-situ stress orientation. High-resolution wellbore images are used as an aid in providing conventional core description and determining orientation. Information obtained using such image is also useful for determining aspects of formation testing, sampling, perforating and other such tasks. For thinly laminated turbidite sands and other sequences, these images are often the only practical method of determining net sand and deposit thicknesses.
A particular challenge has been obtaining micro-resistivity images in wells drilled with non-conductive (commonly referred to as “oil-based”) drilling fluids. Various instruments have been devised to provide images from wells having non-conductive drilling fluids.
One instrument for making resistivity measurements in non-conductive mud is available from Baker Hughes, Incorporated of Houston Tex. The instrument, referred to as an “Earth Imager,” has provided resistivity images in wells drilled with non-conductive drilling fluids.
In some embodiments, the instrument includes six separate pads placed on articulating arms, with each pad including sensor electrodes. A known voltage difference between a return electrode and the sensor electrodes on the pads is used to create a current flow through the formation being imaged. The return electrode and the pad with sensor electrodes are separated by an electrical isolator.
In a typical embodiment, each pad contains a set of eight measuring sensor electrodes together with two focusing electrodes—all surrounded by conductive housings. In some embodiments, the sensor electrodes are referred to as “button electrodes.” To force the measuring sensor current to flow into the formation perpendicular to the instrument near the pad external surface faced to the wellbore wall a control circuitry maintains a zero voltage difference between the focusing electrode and the measuring sensors, this is commonly known as current focusing.
The individual current measurement recorded from each sensor electrode is a function of the formation conductivity and the voltage applied. High resolution images are achieved by sampling at a high rate (for example, about 120 samples per foot) the readings from the forty eight sensor electrodes mounted on the six pads.
Reference may be made to FIG. 1. In FIG. 1, there is shown a depiction of the prior art instrument for performing resistivity imaging in oil based drilling fluids. In this example, the instrument 20 is disposed within a wellbore 11. The instrument 20 includes pads 3 mounted on articulating arms 2. The articulated pads 3 are typically pressed up against a wall of the wellbore 11 to make firm contact therewith. As shown in FIG. 1, current flows from the pads 3 to a return electrode 4. The return electrode 4 is electrically separated from each of the pads 3 by an isolator 5.
In typical embodiments, the instrument 20 operates in non-conductive drilling fluids and provides a current having a frequency f of about 1 MHz. At this frequency f, the capacitive impedance Zc of the drilling fluid drops to a value reasonably small for further measurements and may be determined by Eq. 1:Zc=k(1/(f×C))  Eq. (1),where:                f represents the frequency;        C represents capacitance associated with the gap (standoff) between a respective sensor and a conductive borehole wall, when the gap is filled with drilling fluid;        k represents a constant;Total electrical current I(m) through the sensor electrode, being related by Eq. (2):I(m)=V/(Zc+R)  Eq. (2),where:        V represents a known voltage between sensor electrodes and return electrodes;        I(m) represents a measured current for the sensor electrode; and        R represents a resistor having a value reflecting losses in a formation which are in unique dependency from formation resistivity Rf.        
At a sufficiently high frequency f contribution of the capacitive impedance Zc could become negligible, hence, Eq. 3 is realized:Rf=a(V/I(m))  Eq. (3).where:                a represents a hardware dependent constant typically determined during instrument manufacturing and calibration processes.        
Referring also to FIG. 2, aspects of the prior art pad 3 in relation to a formation 10 are depicted. In FIG. 2A, aspects of current distribution in the formation 10 are shown, where a sensor electrode 22 provides a current into the surrounding formation 10. In FIG. 2B, aspects of capacitive coupling between the pad, the non-conductive mud, and the formation 10 are shown. In FIG. 2C, a frontal view of the pad 3 is depicted. The frontal view also depicts a plurality of the sensor electrodes 22.
As one skilled in the art may understand, the operation theory is oriented to flawless operation and does not account for real-life imperfections. In particular, the above-mentioned pad construction requires high degree of equipotentiality between the sensor electrodes. If the condition has not been met, as one might imagine, the proximity of each of the sensor electrodes 22 to other sensor electrodes 22 can create problems. For example, unavoidable capacitive coupling between neighboring sensor electrodes and/or between sensor electrodes and pad electronics (such as a ground plate) represents a by-pass where a formation current to be measured by each sensor electrode could be partially re-routed. Additionally, non-equipotentiality results in potential difference between buttons that, in turn, induces a current passing through an insulator separating these electrodes. Influence of these in-pad imperfections increases with increasing frequency f and does not include any information about resistivity Rf of the formation. Rather, these imperfections degrade the pad performance, smooth collected images and reduce image resolution.
Therefore, what is needed is a technique for reducing pad imperfections and for minimizing effects of capacitive coupling between sensor electrodes. Preferably, the technique includes ways to correct effects of this capacitive coupling thus providing for restoration of image quality.